is the intraband velocity. The total current is composed from two terms with
\(N_{c}(\mathbf {p},t)\)  and \(P(\mathbf {p},t)\)  which provide two sources for
the generation of harmonics -intraband and interband, respectively.
Interband high harmonics are radiated as a result of pump field-induced
recombination of accelerated electron-hole pairs. Intraband high harmonics
are generated as a result of the independent motion of carriers in their
respective bands. Note that the relative contribution of intraband and
interband high harmonics strongly depends on the nanostructure's material
and pump field parameters. And the contributions of both mechanisms are
essential when the energy gap is small compared with the wave photon energy
[1]}.
